1. Field of the Invention
The present invention relates to a power steering apparatus for providing a hydraulic power assist to the driver's steering effort applied to the steering wheel of an automotive vehicle, and more particularly to a power steering apparatus of the type which is capable of decreasing the hydraulic power assist in accordance with an increase in vehicle speed.
2. Discussion of the Background
As one of power steering apparatuses of this kind, a power steering apparatus of the rotary valve type has been proposed in the U.S. Pat. No. 4,034,825 issued on July 12, 1977, which power steering apparatus comprises a valve housing, an input member in the form of an inner rotating valve sleeve rotatably mounted in the valve housing, an output member arranged coaxially with the input member for relative rotation, and an outer valve sleeve rotatably coupled over the inner valve sleeve and connected integrally with the output member to cooperate with the inner valve sleeve in response to relative rotation between the input and output members thereby to selectively direct fluid under pressure from a source of hydraulic pressure to opposite ends of a hydraulic power cylinder. In this prior proposal, a torsion bar is adapted to interconnect the input and output members for permitting relative rotation between the input and output members, a pair of axial recesses are formed in the inner valve sleeve, and a pair of balls are positioned in the axial recesses and caged in a pair of radial bores in the thick end part of the outer valve sleeve to effect the resistance to relative displacement between the inner and outer valve sleeves. The balls are further arranged to be subject to hydraulic pressure from a secondary hydraulic pump responsive to the vehicle speed. Thus, the balls act to increase the resistance to relative displacement between the inner and outer valve sleeves in proportion to the vehicle speed. In such arrangement, the assembly of the input and output members in the valve housing is complicated due to provision of the torsion bar. Additionally, it is required to maintain the hydraulic pressure applied to the balls at a high level for effecting the resistance to relative rotation between the inner and outer valve sleeves, thus resulting in power loss of the prime mover of the vehicle.